Quad-layer GMR sandwich

ABSTRACT

A magnetoresistive (GMR) sensor includes a substrate and a first trilayer disposed on the substrate. A first spacer layer is disposed on the first trilayer. A first magnetic layer is disposed on the first spacer. A second spacer layer is disposed on the first magnetic layer. A second magnetic layer is disposed on the second spacer layer. A third spacer layer is disposed on the second magnetic layer. A second trilayer is disposed on the third spacer layer and a cap layer is disposed on the second trilayer. The first and second trilayers include, a first ferromagnetic layer, a second ferromagnetic layer and an anti-parallel coupling layer disposed between and in contact with the first and second ferromagnetic layers.

This application claims the benefit of and is a continuation-in-part ofthe provisional application serial No. 60/102,188, entitled “Quad-LayerGMR Sandwich,” filed Sep. 28, 1998 which is incorporated herein.

FIELD OF THE INVENTION

This invention relates generally to a novel structure for a giantmagnetoresistance sensor suitable for high density data applications andto systems which incorporate such sensors. In addition, this inventionfinds utility in any other application in which magnetic field sensingis desired.

BACKGROUND OF THE INVENTION

Computers often include auxiliary memory storage devices having media onwhich data can be written and from which data can be read for later use.A direct access storage device (disc drive) incorporating rotatingmagnetic discs is commonly used for storing data in magnetic form on thedisc surfaces. Data are recorded on concentric, radially spaced trackson the disc surfaces. Magnetic heads including read sensors are thenused to read data from the tracks on the disc surfaces.

In high capacity disc drives, magnetoresistive read sensors, commonlyreferred to as MR heads, are the prevailing read sensors because oftheir ability to read data from a surface of a disc at greater lineardensities than thin film inductive heads. An MR sensor detects amagnetic field through the change in the resistance of its MR sensinglayer (also referred to as an “MR element”) as a function of thestrength and direction of the magnetic flux being sensed by the MRlayer.

The conventional MR sensor operates on the basis of the anisotropicmagnetoresistive (AMR) effect in which an MR element resistance variesas the square of the cosine of the angle between the magnetization inthe MR element and the direction of sense current flow through the MRelement. Recorded data can be read from a magnetic medium because theexternal magnetic field from the recorded magnetic medium (the signalfield) causes a change in the direction of magnetization in the MRelement, which in turn causes a change in resistance in the MR elementand a corresponding change in the sensed current or voltage.

Another type of MR sensor is the giant magnetoresistance (GMR) sensormanifesting the GMR effect. In GMR sensors, the resistance of the MRsensing layer varies as a function of the spin-dependent transmission ofthe conduction electrons between magnetic layers separated by anon-magnetic layer or layers (spacer) and the accompanyingspin-dependent scattering which takes place at the interface of themagnetic and non-magnetic layers and within the magnetic layers.

FIG. 1(a) illustrates a simple, unpinned GMR sensor 100. The simple GMRsensor consists of two magnetic layers 103 and 105 separated by anonmagnetic spacer 104. A cap layer 106 covers one magnetic layer 105and a buffer layer 102 is disposed under the other magnetic layer 103.The entire structure is deposited on a substrate 101. This simpleunpinned GMR sensor 100 provides a limited GMR resulting in a relativelyweak signal.

FIG. 1(b) illustrates the magnetization directions of the simpleunpinned GMR sensor 100 with a bias current 110 flowing into the page.With current bias 110 the magnetization directions of the magneticlayers 105 and 103 are oriented mainly anti-parallel to each other asshown by the arrows.

FIG. 1(c) illustrates the magnetization directions of the simpleunpinned GMR sensor 100 with a bias current 110 flowing into the pageand an external magnetic field 111 applied. When a large enough externalfield 111 is applied, magnetization of the magnetic layers 105 and 103will align with the field direction and the resistance will be low.

The sensors shown in FIGS. 1(a)-(c) are useful for applications such asmagnetic field sensing. Simple unpinned GMR sensors have been used inbridge circuits, however, to operate successfully, i.e., provide adifferential in resistance, one set of simple, unpinned GMR sensors mustbe either shielded or additionally biased. This additional shielding orbiasing adds additional cost and complexity to the bridge circuit.

Therefore, there is a need for a magnetoresistive sensor that providesan increased GMR, resulting in a higher signal output. Also, there is aneed for sensors that provide different field responses based on thecurrent density applied to the sensor without requiring the additionalcomplexity of shielding or biasing.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention there is provided amagnetoresitive (GMR) sensor including a substrate and a first trilayerdisposed on the substrate. A first spacer layer is disposed on the firsttrilayer. A first magnetic layer is disposed on the first spacer. Asecond spacer layer is disposed on the first magnetic layer. A secondmagnetic layer is disposed on the second spacer layer. A third spacerlayer is disposed on the second magnetic layer. A second trilayer isdisposed on the third spacer layer and a cap layer is disposed on thesecond trilayer. The first and second trilayers include, a firstferromagnetic layer, a second ferromagnetic layer and an anti-parallelcoupling layer disposed between and in contact with the first and secondferromagnetic layers.

According to another aspect of the present invention there is provided amagnetoresistive sensor device including a substrate and a firsttrilayer disposed on the substrate. A first spacer layer is disposed onthe first trilayer. A first magnetic layer is disposed on the firstspacer. A second spacer layer is disposed on the first magnetic layer. Asecond magnetic layer is disposed on the second spacer layer. A thirdspacer layer is disposed on the second magnetic layer. A second trilayeris disposed on the third spacer layer and a cap layer is disposed on thesecond trilayer. The first and second trilayers include, a firstferromagnetic layer, a second ferromagnetic layer and an anti-parallelcoupling layer disposed between and in contact with the first and secondferromagnetic layers. The resistance of the magnetoresistive sensor isdependent on the magnitude of an applied bias current.

According to another aspect of the present invention there is provided abridge circuit including a first pair of magnetoresistive structurescoupled to first opposite nodes of a Wheatstone bridge and a second pairof magnetoresistive structures coupled to second opposite nodes of theWheatstone bridge The first pair of magnetoresistive structures has agreater current density than the second pair of magnetoresistivestructures when an external field is applied to the Wheatstone bridge.

According to another aspect of the present invention there is provided adisc drive system including a magnetic recording disc, a magnetoresitivesensor, an actuator for moving the magnetoresitive sensor across themagnetic recording disc and a detection circuitry electrically coupledto the magnetoresitive sensor for detecting changes in resistance of themagnetoresitive sensor caused by rotation of the magnetization axes ofthe first and second laminate layers in response to magnetic fields fromthe magnetically recorded data. The magnetoresistive sensor includes asubstrate and a first trilayer disposed on the substrate. A first spacerlayer is disposed on the first trilayer. A first magnetic layer isdisposed on the first spacer. A second spacer layer is disposed on thefirst magnetic layer. A second magnetic layer is disposed on the secondspacer layer. A third spacer layer is disposed on the second magneticlayer. A second trilayer is disposed on the third spacer layer and a caplayer is disposed on the second trilayer. The first and second trilayersinclude, a first ferromagnetic layer, a second ferromagnetic layer andan anti-parallel coupling layer disposed between and in contact with thefirst and second ferromagnetic layers.

According to another aspect of the present invention there is provided aan apparatus for measuring an external field applied across a Wheatstonebridge. The apparatus includes a four terminal electrical network (A, B,C, D) including a first resistor R₁ connected between network terminals(A) and (B), a second resistor R₂ connected between terminals (B) and(C), a third resistor R₃ connected between the network terminals (C) and(D) and a fourth resistor R₄ being connected across network terminals(A) and (D). The resistors R₁ and R₃ have a first current density when afield is applied across network terminals (A) and (C) and the resistorsR₂ and R₄ have a second current density when the same field is appliedacross network terminals (A) and (C). The second current density is notequal to the first current density. The apparatus also includes meansoperatively coupled across the network terminals (B) and (D) fordetecting a potential across the terminals (B) and (D).

The above, as well as additional objects, features, and advantages ofthe present invention will become apparent in the following detailedwritten description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a fuller understanding of the nature and advantages of the presentinvention, as well as the preferred mode of use, reference should bemade to the following detailed description read in conjunction with theaccompanying drawings. In the following drawings, like referencenumerals designate like or similar parts throughout the drawings.

FIG. 1(a) is a cross-sectional view, not to scale, of a simple GMRsensor.

FIG. 1(b) is a cross section view of the sensor shown in FIG 1(a) withbias current flowing into the page.

FIG. 1(c) is a cross section view of the sensor shown in FIG. 1(a)biased in low resistance state.

FIG. 2 is a simplified drawing of a magnetic recording disc drivesystem.

FIG. 3(a) is a cross-sectional view, not to scale, of a GMR sensoraccording to a preferred embodiment of the present invention.

FIG. 3(b) is a cross-sectional view, not to scale, of the GMR sensorshown in FIG. 3(a) biased in the high resistance state.

FIG. 3(c) is a cross-sectional view, not to scale, of the GMR sensorshown in FIG. 3(a) biased in the low resistance state.

FIG. 4(a) is the transfer curve (%GMR versus applied magnetic field) foran GMR sensor according to the present invention with a low biascurrent.

FIG. 4(b) is a the transfer curve (%GMR versus applied magnetic field)for an GMR sensor according to the present invention with a high biascurrent.

FIG. 5(a) is a comparison transfer curve (%GMR versus applied magneticfield) for a simple GMR sensor with a low bias current.

FIG. 5(b) is a comparison transfer curve (%GMR versus applied magneticfield) for a simple GMR sensor with a high bias current.

FIG. 6 is a simplified drawing of a GMR sensor according to the presentinvention.

FIG. 7(a) is an electrical schematic of a bridge circuit utilizing thepresent invention.

FIG. 7(b) is a physical schematic of the bridge circuit shown in FIG.7(a) utilizing the present invention.

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

The following description is a detailed description of the preferredembodiments presently contemplated for carrying out the presentinvention. This description is made for the purpose of illustrating thegeneral principles of the present invention and is not meant to limitthe inventive concepts claimed herein.

FIG. 2 shows a disc drive system 200 embodying the present invention. Asshown in FIG. 2, at least one rotatable magnetic disc 212 is supportedon a spindle 214 and rotated by a disc drive motor. The magneticrecording media on each disc is in the form of an annular pattern ofconcentric data tracks (not shown) on disc 212.

At least one slider 213 is positioned on the disc 212, each slider 213supporting one or more magnetic read/write heads where the headincorporates the GMR sensor of the present invention. As the discsrotate, slider 213 is moved radially in and out 230 over disc surface sothat heads may access different portions of the disc where desired datais recorded. Each slider 213 is attached to an actuator arm 219 by meansof a suspension 215. The suspension 215 provides a slight spring forcewhich biases slider against the disc surface. Each actuator arm isattached to an actuator 227.

During operation of the disc storage system, the rotation of disc 212generates an air bearing between an air bearing surface on the slider213 (the surface of slider 212 which includes a reading head and facesthe surface of disc is referred to as an air bearing surface (ABS)) anddisc surface which exerts an upward force or lift on the slider 213. Theair bearing thus counter-balances the slight spring force of suspension215 and supports slider slightly above the disc 212 surface by a small,substantially constant spacing during normal operation.

The various components of the disc storage system are controlled inoperation by control signals generated by control unit 229, such asaccess control signals and internal clock signals. Typically, controlunit 229 includes logic control circuits, storage and a microprocessor.The control unit generates control signals to control various systemoperations such as drive motor control signals on line and head positionand seek control signals on line. The control signals on line providethe desired current profiles to optimally move and position slider 213to the desired data track on disc 212.

The above description of a typical magnetic disc storage system, and theaccompanying illustration of FIG. 2 are for representation purposesonly. It should be apparent that disc storage systems may contain alarge number of discs and actuators, and each actuator may support anumber of sliders.

FIG. 3(a) shows a cross-sectional view of a GMR structure 100 accordingto a preferred embodiment of the present invention. The layers of theGMR structure 100 are formed by using a number of techniques including,for example, sputter deposition, ion beam deposition and the like.

The GMR structure is typically formed on a substrate 301. The substrate301 can be any suitable substance, including glass, semiconductormaterial, or a ceramic material. For disc drive applications, thesubstrate 301 may also include a permeable, bottom shield layer (notshown) and a half gap insulator (not shown). A buffer layer 302 isformed on the substrate. The buffer layer 302 is deposited to modify thecrystallographic texture or grain size of the subsequent layers, and maynot be needed depending on the substrate 301. If used, the buffer layer302 may be formed of tantalum (Ta), zirconium (Zr), nickel-iron (Ni—Fe),or Al₂O₃. The buffer layer 302 is preferably about 20 to 80 Angstromsthick and more preferably has a thickness of about 35 Angstroms.

A first trilayer 320 is formed on the buffer layer 302 or substrate 301if no buffer layer is used. The first trilayer 320 consists of a firstferromagnetic layer 332 and a second ferromagnetic layer 330 separatedby an anti-parallel coupling (APC) layer 331.

The second ferromagnetic layer 330 may be formed of nickel-iron,cobalt-iron, nickel-iron-cobalt, and the like materials. The secondferromagnetic layer 330 is formed on the buffer layer 302 or thesubstrate 301. The second ferromagnetic layer 330 preferably has athickness of about 10 to 100 Angstroms and, more preferably has athickness of about 17 Angstroms.

The first ferromagnetic layer 332 may be formed of nickel-iron,cobalt-iron, nickel-iron-cobalt, and the like materials. The firstferromagnetic layer 332 is formed on the APC layer and is in contactwith the spacer 331. The first ferromagnetic layer 332 preferably has athickness of about 10 to 100 Angstroms and, more preferably has athickness of about 35 Angstroms.

The APC layer 331 allows the two ferromagnetic layers 330 and 332 to bestrongly coupled together magnetically in an anti-parallel orientationas shown by the arrows in FIG. 3(b). The APC layer 331 may be formed ofruthenium (Ru), indium and/or rhodium. The APC layer 331 preferably hasa thickness of about 3 to 12 Angstroms, and more preferably has athickness of 9.5 Angstroms.

Typically, the second ferromagnetic layer 330 has a larger magneticmoment than the first ferromagnetic layer 332. This can be accomplishedby depositing a thicker layer of the second ferromagnetic layer 330 thanthe first ferromagnetic layer 332. Alternatively, it may be possible toincrease the magnetic moment of a layer through material selection only.

A first spacer layer 333 is formed on the first trilayer 320. The firstspacer layer 333 is thus formed on and in contact with the firstferromagnetic layer 332. The spacer 333 may be formed of a copper (Cu),gold (Au), silver (Ag) and the like. The first spacer layer 333preferably has a thickness of about 25 to 45 Angstroms and, morepreferably, has a thickness of 32 Angstroms.

A simple GMR structure 322 is formed on the first spacer layer 333. Thesimple GMR structure 322 consists of two magnetic layers 303 and 305separated by a nonmagnetic (second) spacer layer 304. The magneticlayers 303 and 305 may be formed of nickel-iron, cobalt-iron,nickel-iron-cobalt, and the like materials. The magnetic layers 332 and305 preferably have a thickness of about 10 to 100 Angstroms and, morepreferably have a thickness of about 35 Angstroms. The nonmagneticspacer 304 is formed on and in contact with a first magnetic layer 303.The nonmagnetic spacer 304 may be formed of a copper (Cu), gold (Au),silver (Ag) and the like. The nonmagnetic spacer layer 303 preferablyhas a thickness of about 25 to 45 Angstroms and, more preferably, has athickness of about 32 Angstroms. A second magnetic layer 305 is thenformed on and in contact with the nonmagnetic spacer 303.

A third spacer layer 334 is formed on the simple GMR structure 322. Thethird spacer layer 334 is thus formed on and in contact with the simpleGMR structure 322. The third spacer layer 334 may be formed of a copper(Cu), gold (Au), silver (Ag) and the like. The third spacer layer 334preferably has a thickness of about 25 to 45 Angstroms and, morepreferably, has a thickness of about 32 Angstroms.

A second trilayer 321 is formed on the second third layer 334. Thesecond trilayer 321 consists of a first ferromagnetic layer 335 and asecond ferromagnetic layer 337 separated by an anti-parallel coupling(APC) layer 336. The materials and dimensions of this trilayer structureare preferably the same as previously described with reference to thefirst trilayer 320.

A cap layer 306 may be formed of a suitable protective material such astantalum (Ta), Al₂O₃ and the like. The cap layer 306 is deposited on thesecond trilayer 321 to protect the active layers from oxidation,corrosion and the like. The cap layer preferably has a thickness ofabout 20 to 80 Angstroms and, more preferably, has a thickness of about35 Angstroms.

FIG. 3(b) shows a cross-sectional view of the GMR structure according tothe present invention in the zero external field, high resistance state.FIG. 3(b) illustrates the relative magnetization directions with a biascurrent 310 directed perpendicular into the stack as shown. Increasingthe number of layers increased the GMR of the film, resulting in higheroutput signal. It is useful to have the correct orientation of the outermagnetic layers as shown in FIG. 3(b). The outer ferromagnetic layers330 and 337 have a higher moment than the inner ferromagnetic layers 335and 332.

FIG. 3(c) shows a cross-sectional view of the GMR structure shown inFIG. 3(a) biased in a low resistance state. The middle ferromagneticlayers 335, 305, 303, 332 are aligned as shown by the arrows therebygiving the structure a low resistance state. These layers are aligneddue to the small amount of parallel coupling that exists across theinterfaces. The thicker, outer ferromagnetic layers are not aligned butthey do not contribute to the low resistance of the structure since thatis determined by the magnetic orientation of the layers across thespacer layers. A low resistance state may also be achieved when a largeexternal field is applied to the structure.

FIG. 4(a) illustrates a graph of the transfer curve for a resistorpatterned from the GMR structure according to the present invention. Alow bias current, preferably 1 mA, was applied to the structure as shownin FIG. 3(c). FIG. 4(b) illustrates a graph of the transfer curve for aresistor patterned from the GMR structure according to the presentinvention. A high bias current, preferably 20 mA, was applied to thestructure as in FIG. 3(b).

FIG. 5(a) illustrates a graph of the transfer curve for a resistorpatterned from a simple GMR structure (FIG. 1(a)) with a 1 mA biascurrent applied to the structure as in FIG. 1(b). FIG. 5(b) illustratesa graph of the transfer curve for a resistor patterned from a simple GMRstructure (FIG. 1(a)) with a 20 mA bias current was applied to thestructure as in FIG. 1(b).

It can be seen from comparing the graphs of FIGS. 4(a)-(b) and FIGS.5(a)-(b) that the transfer curve of the GMR structure according to thepresent invention is dependent upon the magnitude of the bias currentapplied to the structure. More particularly, the structure has a lowresistance, zero external field state when a low bias current is appliedand it has a high resistance, zero external field state when a high biascurrent is applied. The simple GMR structure shown always has a highresistance, zero external bias field no matter what bias current isapplied.

Field Sensors

FIG. 6 shows a field sensor according to a preferred embodiment of thepresent invention. The sensor 600 is formed as a GMR structure accordingto the present invention as already described. If a constant currentl_(in) 602 is applied to the sensor, a constant current I_(out) 604results. So, for example, if I_(in) is a small current so is I_(out).For I_(insmall) the potential difference across the sensor 606 ismeasured and it is constant. If an external field 608 is then applied,the resistance of the sensor 600 changes thereby causing a change in thesensed potential. Then, if a larger current is applied, I_(inlarge), andan external field is applied, a change in resistance of the sensor 600will occur thereby causing a change in the sensed potential. Because thesensor is constructed according to the present invention, the change inresistance of the sensor for a large input current is different than thechange in resistance of the sensor for a small input current. Thus, themagnitude of the input current can be determined by sensing the changein potential across the sensor 606, i.e., the change in resistance ofthe sensor.

A bridge circuit, preferably a Wheatstone bridge circuit, in which giantmagnetoresistive structures according to the present invention are usedas resistors 704, 705 is shown in schematic form in FIGS. 7(a)-(b). Avoltage in 701 and ground 702 is shown connected between two opposingnodes of the bridge at each of which two of the four GMR resistors areelectrically connected as is well known.

More particularly, the bridge circuit has a four terminal electricalnetwork (A, B, C, D). A first resister R₁ is connected between networkterminals (A) and (B), a second resister R₂ is connected between networkterminals (B) and (C), a third resistor R₃ is connected between thenetwork terminals (C) and (D) and a fourth resister R₄ is connectedbetween network terminals (A) and (D). The first and third resisters R₁and R₃ preferably have the same current density when a field is appliedacross network terminals (A) and (C) and resistors R₂ and R₄ preferablyhave the same current density which is different from the currentdensity of the first and third resistors R₁ and R₃.

The bridge can be used as a sensor to determine the magnitude of anapplied external field. For example, a constant voltage is V_(in) atterminal 701. An output V_(out) is detected, and, at zero external fieldwill equal zero. But if an external field is applied, the resistance ofresistors R₁ and R₃ changes differently than the resistance of resistorsR₂ and R₄ so that V_(out) does not equal zero. By detecting the changein V_(out), the magnitude of the external field can be determined.

Thus, to use the bridge as a sensor, the variation in resistance in anapplied field of resistors R₁ and R₃ had to be different than thevariation in resistance of resistors R₂ and R₄. In the past, usingsimple GMR structures, this was accomplished by shielding one set of theresistors. Another way that was used to accomplish this was to bias oneset of resistors so that its transfer curve would be shifted so that,the resistance of those resistors would go either up or down. One way toaccomplish this bias is to use an external field. This, of course, hadthe disadvantage of requiring additional power and circuit complexity toshift the resistors transfer curves.

The GMR structure according to the present invention is used and thefirst set of resistors R₁ and R₃ is patterned differently from thesecond set of resistors R₂ and R₄. More particularly, the width ofresistors R₁ and R₃ is made wider so that the current density throughresistors R₁ and R₃ is lower than the current density through R₂ and R₄since current density is inversely proportional to width. FIG. 7(b)illustrates the physical configuration of a Wheatstone bridge circuitaccording to the present invention. Preferably, the ratio of widths ofR₁, R₃ to R₂, R₄ ranges from about 2:1 to about 20:1.

Experiment

The stacks schematically illustrated in FIG. 1(a) and FIG. 3(a) weredeposited in an S-gun sputter deposition system. The NiFeCo layers wereco-sputtered from NiFe and CoFe targets; all other layers were depositedfrom single targets. Prior to deposition, photoresist was patterned andthe devices were defined using a liftoff process.

Quad layer GMR films consisting of a total of four magnetic layers; twosimple ferromagnetic layers and two synthetic antiferromagnet layers,were deposited and patterned into devices (FIG. 3(a)). The syntheticantiferromagnet layers are in the form of a trilayer structure of twoferromagnetic layers separated by a spacer. In a preferred embodiment,the outer layers 330, 337, i.e., the layers facing away from the centerof the stack, were designed to be thicker than the inner layers 332,335, so that a high enough sense field would produce the highestresistance state for the stack as a whole, with antiparallelmagnetization orientation of the magnetic layers across all three spacerinterfaces 333, 304, 334. Upon application of an external magnetic fielddown the length of the stripe, resistance decreases to an intermediateresistance state.

It was found that the device formed according to the present inventionbehaved differently depending on the magnitude of the bias currentapplied. More particularly, if a low bias current is applied, a low zerofield bias state, i.e. when no external field is present, is achieved.The resistance of the structure increases as the strength of theexternal field increases. If a large bias current is applied, a highzero field bias state is achieved. The resistance of the structuredecreases as the strength of the external field increases. A simple GMRsensor such as that shown in FIG. 1(a) always has a high zero field biasstate no matter what bias current is applied.

Transfer curve data were taken for approximately 6 μm wide devicespatterned from both the Quad layer GMR (FIG. 3(a)) and simple GMR (FIG.1(a)) structures with a fixed bias current applied down a strip line.The material easy axis was oriented across the stripe, and the externalfield was applied down the length of the stripe to saturate the devices.Data for the Quad layer structure FIG. 3(a) is shown in FIGS. 4(a)-(b).Depending upon the current density through the device, the zero fieldbias state of the patterned resistors is either low or high. A lowcurrent density results in the low bias state and resistance increasesas the applied field increases. In order to achieve a low resistance,zero field bias state, presumably several of the ferromagnetic layersare aligned essentially parallel to one another across the Cu spacers. Asmall amount of parallel or “orange peel” coupling across the Cu spacersis typical for sandwich structures with Cu in this thickness range. Ascurrent through the device is increased, the field generated overcomesthe weak parallel coupling. At a high enough current density, the deviceis biased high at zero field, and resistance decreases with appliedfield strength.

The data indicate that the Quad layer GMR material (FIG. 3(a))could beused for current sensing, as well as magnetic field sensing,applications. In addition to the flexibility of the zero-field biasstate, another advantage of the Quad layer GMR structure (FIG. 3(a)) isthat devices may be operated with relatively low current density ascompared to simple GMR sandwich films (FIG. 1(a)). FIG. 4(a) shows datafor a resistor patterned from the Quad layer GMR sandwich material (FIG.3(a)) that exhibits about a 1.7% GMR signal with 1 mA of bias current Aresistor with the same linewidth patterned from the more conventionalGMR sandwich stack (FIG. 1(a)), shown in FIG. 5(a), exhibits only about0.3% GMR tested under the same conditions.

The above specification, examples and data provide a completedescription of the manufacture and use of the composition of theinvention. Since many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended.

I claim:
 1. A magnetoresistive sensor, comprising: a substrate; a firsttrilayer disposed on the substrate; a first spacer layer disposed on thefirst trilayer; a first magnetic layer disposed on the first spacerlayer, a second spacer layer disposed on the first magnetic layer; asecond magnetic layer disposed on the second spacer layer; a thirdspacer layer disposed on the second magnetic layer; a second trilayerdisposed on the third spacer layer; and a cap layer disposed on thesecond trilayer; wherein, the first and second trilayer comprises: afirst ferromagnetic layer; a second ferromagnetic layer; and ananti-parallel coupling layer disposed between and in contact with thefirst and second ferromagnetic layers.
 2. The sensor of claim 1, whereinthe second spacer layer is a nonmagnetic layer.
 3. The sensor of claim1, further comprising a buffer layer disposed on the substrate and incontact with the first trilayer.
 4. The sensor of claim 3, wherein thebuffer layer comprises a material selected from the group consisting ofTa, Zr, Ni—Fe, and Al₂O₃ and mixtures thereof.
 5. The sensor of claim 3,wherein the buffer layer has a thickness of about 20 to about 80Angstroms.
 6. The sensor of claim 1, wherein the second ferromagneticlayer is thicker than the first ferromagnetic layer.
 7. The sensor ofclaim 1, wherein the first and second ferromagnetic layers comprise amaterial selected from the group consisting of Ni, Fe, Co and mixturesthereof.
 8. The sensor of claim 1, wherein the first and secondferromagnetic layers have a thickness of about 10 to about 100Angstroms.
 9. The sensor of claim 1, wherein the anti-parallel couplinglayer comprises a material selected from the group consisting of Ru, Ir,Rh and mixtures thereof.
 10. The sensor of claim 1, wherein theanti-parallel coupling layer has a thickness of about 3 to about 12Angstroms.
 11. The sensor of claim 1, wherein the first spacer layercomprises a material selected from the group consisting of Cu, Au, Agand mixtures thereof.
 12. The sensor of claim 1, wherein the firstspacer layer has a thickness of about 25 to about 45 Angstroms.
 13. Thesensor of claim 1, wherein the first and second magnetic layers comprisea material selected from the group consisting of Ni, Fe, Co and mixturesthereof.
 14. The sensor of claim 1, wherein the first and secondmagnetic layers have a thickness of 10 to about 100 Angstroms.
 15. Thesensor of claim 1, wherein the nonmagnetic spacer comprises a materialselected from the group consisting of Cu, Au, Ag and mixtures thereof.16. The sensor of claim 1, wherein the nonmagnetic spacer has athickness of about 25 to about 45 Angstroms.
 17. The sensor of claim 1,wherein second spacer layer comprises a material selected from the groupconsisting of Cu, Au, Ag and mixtures thereof.
 18. The sensor of claim1, wherein the second spacer layer has a thickness of about 25 to about45 Angstroms.
 19. The sensor of claim 1, wherein the cap layer comprisesa material selected from the group consisting of Ta, A1 ₂O₃ and mixturesthereof.
 20. The sensor of claim 1, wherein the cap layer has athickness of about 20 to about 80 Angstroms.
 21. A magnetoresistivesensor device, comprising: a substrate; a first trilayer disposed on thesubstrate; a first spacer layer disposed on the first trilayer; a firstmagnetic layer disposed on the first spacer layer; a second spacer layerdisposed on the first magnetic layer; a second magnetic layer disposedon the second spacer layer; a third spacer layer disposed on the secondmagnetic layer; a second trilayer disposed on the third spacer layer;and a cap layer disposed on the second trilayer; wherein, the first andsecond trilayer comprises: a first ferromagnetic layer; a secondferromagnetic layer; and an anti-parallel coupling layer disposedbetween and in contact with the first and second ferromagnetic layers;and wherein, the resistance of the magnetoresistive sensor is dependentupon the magnitude of an applied bias current.
 22. The sensor device ofclaim 21, wherein the second spacer layer is a nonmagnetic layer.
 23. Abridge circuit, comprising: a first pair of magnetoresistive structurescoupled to first opposite nodes of a Wheatstone bridge; a second pair ofmagnetoresistive structures coupled to second opposite nodes of theWheatstone bridge; wherein the first pair of magnetoresistive structureshas a greater current density than the second pair of magnetoresistivestructures when an external field is applied to the Wheatstone bridge.24. The bridge circuit of claim 23, wherein the first pair ofmagnetoresistive structures is larger than the second pair ofmagnetoresistive structures.
 25. The circuit of claim 24, wherein thefirst pair of magnetoresistive structures to the second pair ofmagnetoresistive structures ranges has a width ratio from about 1:2 toabout 1:20.
 26. A disc drive system, comprising: a magnetic recordingdisc; a magnetoresitive sensor comprising: a substrate; a first trilayerdisposed on the substrate; a first spacer layer disposed on the firsttrilayer; a first magnetic layer disposed on the first spacer layer; asecond spacer layer disposed on the first magnetic layer; a secondmagnetic layer disposed on the second spacer layer; a third spacer layerdisposed on the second magnetic layer; a second trilayer disposed on thethird spacer layer; and a cap layer disposed on the second trilayer;wherein, the first and second trilayer comprises: a first ferromagneticlayer; a second ferromagnetic layer; and an anti-parallel coupling layerdisposed between and in contact with the first and second ferromagneticlayers; an actuator for moving the magnetoresitive sensor across themagnetic recording disc; and a detection circuitry electrically coupledto the magnetoresitive sensor for detecting changes in resistance of themagnetoresitive sensor caused by rotation of the magnetization axes ofthe first and second laminate layers in response to magnetic fields fromthe magnetically recorded data.
 27. The sensor device of claim 26,wherein the second spacer layer is a nonmagnetic layer.
 28. An apparatusfor measuring an external field applied across a Wheatstone bridge, theapparatus comprising: a four terminal electrical network (A, B, C, D)including a first resistor R₁ connected between network terminals (A)and (B), a second resistor R₂ connected between terminals (B) and (C), athird resistor R₃ connected between the network terminals (C) and (D)and a fourth resistor R₄ being connected across network terminals (A)and (D); wherein resistors R₁ and R₃ have a first current density when afield is applied across network terminals (A) and (C) and the resistorsR₂ and R₄ have a second current density when the same field is appliedacross network terminals (A) and (C) wherein the second current densityis not equal to the first current density; means operatively coupledacross the network terminals (B) and (D) for detecting a potentialacross the terminals (B) and (D).
 29. The apparatus of claim 28, whereinthe first current density of resistors R₁ and R₃ increases as the fielddecreases and the second current density of resistors R₂ and R₄decreases as the field increases.
 30. The apparatus of claim 28, whereinthe first, second, third and fourth resistors are magnoresistivesensors, comprising: a substrate; a first trilayer; disposed on thesubstrate; a first spacer layer disposed on the first trilayer; a firstmagnetic layer disposed on the first spacer layer; a second spacer layerdisposed on the first magnetic layer; a second magnetic layer disposedon the second spacer layer; a third spacer layer disposed on the secondmagnetic layer; a second trilayer disposed on the third spacer layer;and a cap layer disposed on the second trilayer; wherein, the first andsecond trilayer comprises: a first ferromagnetic layer; a secondferromagnetic layer; and an anti-parallel coupling layer disposedbetween and in contact with the first and second ferromagnetic layers.